The present invention relates to an image heating apparatus used with an image forming apparatus such as a copying machine, a printer, and the like. In particular, it relates to an image heating apparatus in which a second member, such as a piece of film or the like, slides on a first member.
For ease of description, the present invention will be described with reference to a fixing apparatus, with which an image forming apparatus, for example, a copying machine, a printer, or the like, is provided, and which thermally fixes a toner image to a piece of recording medium.
In an image forming apparatus, an image (unfixed toner image) is formed through an optional image forming process, for example, an electrophotographic process, an electrostatic recording process, a magnetic recording process, or the like, and is directly placed, or indirectly placed (transferred) onto a piece of recording medium (transfer sheet, electro-fax sheet, electrostatic recording sheet, OHP sheet, printing sheet, format sheet, and the like). The unfixed toner image placed on the recording medium is thermally fixed, as a permanent image, to the surface of the recording medium by a fixing apparatus. As for such a fixing apparatus, a heat roller type fixing apparatus has been widely used.
In recent years, a film heating type fixing apparatus which employs a heater has been put to practical use, in view of quick starting and energy saving. Also, an film heating type fixing apparatus which uses an electromagnetic induction heating system has been proposed.
A film heating type fixing apparatus has been proposed in various Japanese official patent gazettes, for example, Japanese Laid-open Patent Application Nos. 313,182/1985, 157,878/1990, 44,075/1992, and 204,980/1992.
Generally speaking, a film heating type fixing apparatus is a fixing apparatus which comprises a ceramic heater as a heating member, a pressure roller as a pressure applying member, and a piece of heat resistant film (fixing film) placed between the ceramic heater and pressure roller. The ceramic heating member is pressed upon the pressure roller, with the fixing film being between them, to form a fixing nip. In operation, a piece of recording medium, on or onto which on unfixed toner image, that is, the image to be fixed, has been formed or transferred, is introduced into the fixing nip, and put through the nip together with the fixing film. As the recording medium is put through the fixing nip, the heat from the ceramic heating member, along with the pressure from the pressure roller, is applied to the recording medium through the fixing film. As a result, the unfixed toner image on the recording medium is fixed to the recording medium.
This film heating system makes it possible to realize an on-demand type fixing apparatus, which employs a ceramic heater, and a piece of film with low thermal capacity, and in which electrical power is applied to the ceramic heater to obtain the fixing temperature, only during the image formation by an image forming apparatus. An on-demand type fixing apparatus is advantageous in that the time is short from when the image forming apparatus is turned on to when the image forming apparatus becomes ready for image formation (it starts quickly), and is drastically smaller in the power consumption during a standby period (energy saving).
In Japanese Laid-open Patent Application No. 114,276/1995, an induction heating type fixing apparatus is disclosed, in which electrical current is induced in a piece of fixing film to generate heat therein (Joule heat). With this arrangement, heat is directly generated within the film, with the use of the induction current generated within the film, making it possible to realize a fixing process with a much higher efficiency than those of conventional fixing processed.
As for the method for driving the fixing film of a film heating type fixing apparatus, there has been devised a method in which the fixing film is rotationally driven by driving a pressure roller (pressure roller driving method), a method in which the fixing film is rotationally driven by a driver roller placed within the loop of the fixing film, and the like.
As for the pressure applying method, a generally used method is such that the fixing film is pinched between a film supporting member, such as the heater, or a film guide, disposed within the loop of the fixing film, and pressure is generated by a spring or the like placed, in the compressed state, between the pressure roller or the film supporting member (or a pressure application stay placed within the loop of the fixing film), and a spring seat.
However, there has been one object to be accomplished regarding a fixing apparatus which employed a film heating system such as the one described above, which was to improve the fixing film in terms of its slidableness against the film supporting member during the rotational driving of the fixing film.
More specifically, the object is to solve the problem that, if the pressure within the fixing nip is set relatively high to assure that a toner image is properly fixed to high gross paper, thick paper, or the like, the driving roller tends to fail to rotate, and therefore, the fixing film is not driven, during the starting-up of a fixing apparatus in a low temperature environment or the like. It is thought that this problem occurs because, not only is the higher pressure applied between the pressure roller and film supporting member, but also the slidableness of the fixing film against the film supporting member reduces in a low temperature environment, making it impossible for the amount of the torque which the driving roller can generate, to catch up with the torque necessary to rotationally drive the fixing film.
When the film failed to be moved, various problems occurred. For example, it took a longer time for the fixing apparatus to warm up, and/or the film was sometimes damaged due to the local increase of film temperature.
Further, the motor for driving the driving roller was subjected to an extra amount of external load generated on the film was prevented from being rotationally driven, and as a result, the motor sometimes failed. It was possible to employ a motor with a higher torque. But, such a measure led to the increase in the apparatus size and cost, and therefore, it was not the better way.
The primary object of the present invention is to provide an image heating apparatus capable of assuring that a second member smoothly slides against a first member even while the image heating apparatus is started up.
According to on aspect of the present invention, an image heating apparatus comprises a first member, a second member which slides against said first member, a third member which forms a nip against said first member, with the second member being disposed between the first and third members, and a pressure controlling means capable of providing two levels of pressure, that is, a first level, and a second level greater than the first level, in said nip, wherein an image borne on a piece of recording medium is heated while the piece of recording medium, on which an image has been borne, is conveyed through said nip, while being pinched between said second and third members, and wherein when said second member begins to be moved from its still state as an image heating operation is started, the pressure in said nip is set at the first level, and then, is switched to the second level.
These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings.